Cook elbow pasta to el dente. Drain. Set aside.

While the pasta cooks, place the butter into a medium to large sauce pan and melt over medium to medium low heat.

Add the flour and whisk to cook for 1-2 minutes (until it smells like pie dough).

Slowly add the milk while whisking. Adjust heat as needed.

Add the sour cream and heat through while whisking to get a thick, smooth and creamy sauce.

Add salt and pepper and then, both cheeses, one at a time. Stir to melt.

Then, add the pesto, stir.

Add the pasta and stir to combine.

Add more milk, as needed to reach your desired consistency-just a tablespoon at a time.

Top with dried parsley, if desired and serve immediately. Enjoy with a salad or as the side to a main course, like steak or burgers.